The present invention relates to an in-plane switching type active driving liquid crystal display device, wherein a liquid crystal is operated by an electric field which extends in a plane substantially parallel to the substrate; and, in particular, the invention relates to an in-plane switching type active driving liquid crystal display device using a liquid crystal having a negative dielectric anisotropy.
A liquid crystal display device displays images by utilizing the optical change of a liquid crystal layer generated by changing the orientating direction of the liquid crystal when an electric field is applied to the liquid crystal interposed between a pair of substrates.
The conventional active driving liquid crystal display device is represented by a twisted nematic (TN) type liquid crystal display device. In accordance with the TN type liquid crystal display device, electrodes are provided, respectively, on the upper substrate and the lower substrate between which the liquid crystal is disposed; the electrodes are arranged so that an electric field between the electrodes is applied to the liquid crystal in a direction perpendicular to the substrate plane; and the image display is performed by utilizing the rotating optical characteristic of the liquid crystal layer. One of the serious problems inherent in the TN type active driving liquid crystal display device is the narrowness of the viewing angle.
In order to maintain a high display quality(contrast ratio), a high voltage holding rate is necessary. In order to achieve the high voltage holding rate, use of a liquid crystal having an extremely high specific resistance is indispensable.
The liquid crystal used for the liquid crystal display device is generally composed of a composition of various liquid crystal molecules consisting of high polar liquid crystal molecules for providing a designated dielectric anisotropy, and low polar liquid crystal molecules for adjusting the liquid crystal characteristics other than the dielectric anisotropy, such as viscosity, the temperature range of the liquid crystal condition, and others.
In order to realize such a high specific resistance value, it is necessary to restrict the kind of liquid crystal molecule to be used with a view toward making the liquid crystal highly resistant, that is, in view of the readiness of purification. Accordingly, use of a liquid crystal molecule having a cyano group in its molecule as the high polar liquid crystal molecular component tends to be avoided as much as possible. As a result, the range of selection of the liquid crystal molecule component, in particular, the range of selection of a high polar liquid crystal molecular component, becomes generally quite narrow.
On the other hand, currently, an in-plane switching (IPS) type liquid crystal display device has been developed, wherein comb-teeth electrodes are used; the liquid crystal is operated in response to an electric field that has a major component disposed in parallel to the substrate; and display is performed by utilizing the birefringence of the liquid crystal. An example of this IPS type of display device has been disclosed in JP-A-6-222397 (1994).
The IPS type liquid crystal display device has a wider viewing angle than the TN type liquid crystal display device, because of in-plane switching of the liquid crystal molecule, and it has other advantages, such as a low load capacity and others. Therefore, currently, the technology related to the IPS type liquid crystal display device has been rapidly progressing as a new active driving type liquid crystal display device which can be substituted for the conventional TN type liquid crystal display device.
In accordance with the IPS system, a more perfect in-plane switching can be realized when the liquid crystal has a negative dielectric anisotropy, in comparison with a case when the liquid crystal has a positive dielectric anisotropy, as disclosed in M. Oh-e, M. Yoneya, and K. Kondo, Journal of Applied Physics, 1997, Vol. 82, No. 4, 528-535. Achievement of such a perfect in-plane switching makes it possible to broaden the viewing angle of the liquid crystal display device, including half tone display, perfectly.
Accordingly, in view of the above advantage, the liquid crystal used in the IPS system desirably has a negative dielectric anisotropy.
However, as long as active driving is used, the high voltage holding rate is an indispensable condition even in the IPS system. A remarkably high resistance is required for the liquid crystal as well as the conventional TN system. In this case, a liquid crystal of negative dielectric anisotropy may generate a new problem.
The problem relates to variation of the liquid crystal molecule. That is, in order to realize the negative dielectric anisotropy of the liquid crystal molecule, the dielectric constant in a direction of the minor axis perpendicular to the longitudinal direction of the liquid crystal molecule must be made larger than that in the longitudinal direction of the liquid crystal molecule. However, in order to realize the above condition, a large restriction exists in the molecular design, because the conjugated group can not be extended arbitrarily in the minor axis direction of the liquid crystal molecule. Conventionally, the active driving TN type liquid crystal display device uses a liquid crystal of positive dielectric anisotropy, and a liquid crystal of negative dielectric anisotropy has not been called for in particular. Therefore, the technology concerning a liquid crystal of negative dielectric anisotropy has not been developed yet, and a variation of the available liquid crystal molecule is extremely small.
Therefore, the range of the selection of the liquid crystal molecule for a liquid crystal of negative dielectric anisotropy is very small; in particular, the kinds of high polar liquid crystal molecule, which can be used for providing a desirable dielectric anisotropy, is extremely small. Accordingly, when a liquid crystal of negative dielectric anisotropy is used, there is a possibility that the liquid crystal display device will not be able to attain desired characteristics. In particular, there is a concern that that the driving voltage will need to be increased significantly, because the dielectric anisotropy can not be increased sufficiently.
However, currently, a new finding on the IPS type liquid crystal display device was disclosed in M. Oh-e, Y. Umeda, M. Ohta, S. Aratani, and K. Kondo, Japan Journal of Applied Physics, Vol. 36 (1997), Part 2, No. 8A, pp. L1025-L1028. By using a component of electric field parallel to the substrate, the IPS system can utilize substrates and an orientation layer between the substrates and the liquid crystal as a storage capacitor, and the liquid crystal display device can maintain a high voltage holding rate. In comparison with a conventional TN system, the decrease in the voltage holding rate is small, even if a liquid crystal having a low specific resistance is used.
Accordingly, it was revealed that such an IPS system provides a possibility to broaden the range of selection of usable liquid crystal molecules, in particular, the range of selection of a high polar component liquid crystal, in comparison with conventional TN system.
In view of the above, the inventors have achieved the present invention disclosed hereinafter by studying the use of a liquid crystal of negative dielectric anisotropy in an IPS system.
Accordingly, the present invention has the object of providing a liquid crystal display device using a liquid crystal of negative dielectric anisotropy, which is usable in an in-plane switching type liquid crystal display device.
The liquid crystal display device in accordance with a first aspect of the present invention is able to solve the above problems by providing:
A liquid crystal display device comprising a pair of substrates; a liquid crystal layer held between the pair of substrates; and pixel electrodes, common electrodes, and active elements arranged on at least one of the pair of substrates, the liquid crystal molecules in the liquid crystal layer being controlled by applying a voltage between a pixel electrode and a common electrode; wherein, the liquid crystal layer has a negative dielectric anisotropy by containing liquid crystal molecules having a dicyanobenzene structure in its molecular structure.
The liquid crystal display device in accordance with a second aspect of the present invention provides:
A liquid crystal display device comprising a pair of substrates; a liquid crystal layer held between the pair of substrates; and pixel electrodes, common electrodes, and thin film transistors arranged on at least one of the pair of substrates the liquid crystal molecules in the liquid crystal layer being controlled by applying a voltage between a pixel electrode and a common electrode; wherein, the liquid crystal material in the liquid crystal layer has a negative dielectric anisotropy by containing liquid crystal molecules which contain a cyano structure in its molecular structure; and at least one of the pixel electrode and the common electrode is composed of a transparent conductive film.
Furthermore, at least a part of the pixel electrode and the common electrode of the liquid crystal display device are overlapped via an insulating film to form an additive capacitor.
The transparent conductive film of the liquid crystal display device is an Indium-Tin-Oxide (ITO) film.
The insulating film held between the pixel electrode and the common electrode of the liquid crystal display device is made of a material selected from the group consisting of silicon nitride, titanium oxide, and silicon oxide.
The liquid crystal material in the liquid crystal layer of the liquid crystal display device is composed of liquid crystal molecules, including liquid crystal molecules having a negative dielectric anisotropy by having a dicyanobenzene structure in its molecular structure.